1. Field of the Invention
This invention relates to a fuel assembly for a nuclear reactor, the fuel assembly being of the kind comprising a housing which receives, in a removable manner, a bundle of elongated fuel rods.
2. Description of Prior Art
The housing of a fuel assembly of the kind referred to is normally in the form of a tubular container having a length of several meters. Its cross-section may, for example, be circular or square and it contains a large number of elongated tubular fuel rods, arranged parallel to one another in a certain specified, normally symmetrical, pattern. The fuel rods are held fixed in this pattern by spacer means comprising a plurality of so-called spacers, for example of the kind disclosed in U.S. Pat. No. 3,405,033. The spacers are arranged in spaced relationship to each other along the vertically arranged bundle of fuel rods, and are held in position along the bundle by a certain amount of self-resilience against the individual fuel rods. However, because of the neutron irradiation, the spring force exerted by the spacers decreases and after a few years it is only a fraction of the original spring force. It may then occur that the spacers, under the influence of the flow through the fuel assembly, are displaced upwards, or slide downwards under the influence of gravity, thus leaving long sections of the fuel rod bundle completely unsupported. To avoid this and fix the spacers in the vertical direction, special spacer holder rods have been arranged in parallel with the fuel rods. Each spacer holder rod is provided with stop means embossed at specified distances from each other along the rod, and individual spacers rest against these stop means. In this way, the spacers are retained in the desired position and are prevented from moving along the fuel rods. However, a particular drawback of these spacer holder rods is that they take up space in the fuel rod bundle which could otherwise be used to receive fuel rods, thus reducing the operating economy of the reactor.
Another way of maintaining the distances between the spacers has been to place spacing sleeves between the spacers, these spacing sleeves being arranged to surround the control rod tubes. A further known method is to fix the spacers permanently to the wall of the surrounding housing. However, this involves the complication that the bundle of fuel rods, together with the applied spacers, cannot be withdrawn from the housing as a single unit when refuelling is necessary.
The present invention aims to provide a fuel assembly for a nuclear reactor which does not have the above-mentioned drawbacks of the known fuel rod assemblies.